AWWA WQTC59009

Growing concerns over the presence of perchlorate in drinking water sources has promotedresearch on treatment methods to remove this compound. Biological treatment usingpacked bed reactors, such as biological activated carbon (BAC), has been proven to be aviable option. Perchlorate is removed by perchlorate reducing bacteria (PRB) growing asbiofilms in BAC. PRB and other heterotrophs constitute the biofilm. PRB can utilizeperchlorate as electron acceptor. However, PRB can also use oxygen when available.Therefore, maintaining low oxygen concentrations is important to accomplish removal ofperchlorate.Backwashing in biological filters serves two purposes. First, regular backwashing isnecessary to prevent clogging and excessive pressure drops across the filter and, second,backwashing is necessary to maintain an active biofilm and ensure good external masstransfer of the contaminant to the biofilm (Hozalski and Bouwer, 1998). A previous studyhas shown that biomass in BAC filters is retained both in the form of thin biofilmattached to the surface of the carbon fibers and also in the form of large microbialaggregates (up to several hundred micrometers) that accumulate in the inter-particle spacebetween the fibers (Choi et al., 2003). These large aggregates can be viewed as thickbiofilms that are diffusion limited in contrast to the thin biofilms on the carbon fibers thatare likely to be fully penetrated. Backwashing can be expected to washout a large fractionof these loosely attached aggregates (Delahaye et al., 1999). Biofilm directly attached to thecarbon fibers is more resistant to shear and only a smaller fraction of this directly attachedbiomass is expected to be lost during backwashing. Thus, backwashing should not onlyreduce the total biomass in the system, but also is expected to change the relative amount ofthick biofilms (i.e., large microbial aggregates) to thin biofilms (directly attached to thesurface of the carbon fibers).This paper demonstrates that large aggregates are important to maintainperchlorate removal in cases of increased influent oxygen concentrations. Mass transferlimitations in the large aggregates cause anaerobic zones to develop in the center of theseaggregates, allowing for perchlorate removal to take place. With increased oxygenconcentrations in the bulk phase, the thin biofilms on the surface of the carbon fibers willbe fully penetrated and will not allow for perchlorate removal to occur. Thus, thehypothesis tested in this paper is that vigorous backwashing of a BAC filter has only alimited influence on perchlorate removal as long as oxygen concentrations in the bulkphase are low. However, after backwashing, BAC filters are more vulnerable to increasedbulk phase oxygen concentrations due to the loss of the majority of the large microbialaggregates. To test this hypothesis, a lab-scale BAC filter was operated and perchlorateremoval was monitored before and after intensive backwashing. Includes 6 references, figures.

Product Details

Edition:

Vol. - No.

Published:

11/02/2003

Number of Pages:

6

File Size:

1 file , 250 KB